Z01 HD 001123 LCE and Z01 HD 001124 LCE are companion projects that together investigate auditory communication in primates. The overall goal of these studies is to provide a comprehensive understanding of primate auditory communication in terms of development, neural mechanisms, endocrine factors, and social context. Two non-human primates, the squirrel monkey and common marmoset, are the main subjects of study, with additional data collected from other species where appropriate. Prior work in this project has shown that production of sounds that are the functional and acoustic equivalents of cry sounds in human infants are mediated by limbic cortex located along the anterior midline of the frontal lobe of the cerebral cortex, and that single neural elements in the auditory cortex (superior temporal gyrus) are particularly responsive to subtle differences in the acoustic structure of species-specific vocalizations, suggesting an important role in mediating individual differences (vocal signatures). New findings this year: The gene c-fos and its protein product Fos, have been used to mark neural regions of increased activity following a period of robust behavioral activity. A study of c-fos expression in infant marmoset monkey brains was conducted. The paradigm involved separating individuals for 30 minutes, recording the ?isolation calls? made during this separation, returning them to their home cage for 1 hour, then euthanizing them and processing their fixed brains for Fos using immunocytochemistry. Seven individuals were used, 2 at 1, 2, and 3 months of age and 1 control (not separated). Using a microscope and motorized stage, sections were imaged at 100 times magnification, then digitized in small increments so that 200-700 images per section were saved. Each image was counted using a software program that selects strongly labeled cells while skipping artifacts and weak labels. Strong label was found in many parts of the infant brain. Of particular note is that regions found to be strongly labeled in adult monkeys were also strongly labeled in infants. In particular, the anterior cingulate gyrus, midline cortex in the frontal lobe, had numerous labelled cells even at 1 month of age. These results indicate that c-fos immunocytochemistry is a method that can demonstrate functional involvement of brain regions also active in adults using the same behavioral paradigm. In a second study, further work was done on a brain atlas for the marmoset. Labels (structure names) from a female brain were checked against an existing but out-of-print marmoset brain atlas and, in addition, photographs of sections from a male marmoset brain were taken, along with photographs of sections cut in the horizontal plane. Our collaboration with Dr. Afonso Silva of NINDS was continued, this year mapping images from the histological atlas against MRI images collected from marmosets in Dr. Silva's laboratory. The histological atlas permitted detailed identification of specific structures in the MRI images.